1. Generator Application and Sizing
By: Arnie De Guzman

2. Generator Duty Cycle/Application
Power rating - The power of the generating set is the power output available for consumer loads at the generating set terminals excluding the electrical power absorbed by the essential independent auxiliaries.
• What is the average load factor?
• What is the maximum required load?
• How many hours per year will the generator sets run?
• Will the generator sets be run isolated from or in parallel with the utility?

3. Limited-Time running Power (LTP)
Limited-time running power is defined as the maximum power available, under the agreed operating conditions, for which the generating set is capable of delivering for up to 500 h of operation per year.
Example is, if there is a unit running continuous and a spare genset is to run during maintainance.

4. Emergency Standby Power (ESP)
Typical usage of 50 hours per year with a maximum of 200 hours per year with varying loads. Average variable load factor is 70% of ESP rating. Emergency standby power is the maximum power available during a variable load sequence in the event of a utility power outage or under test conditions for up to 200 hours a year while maintaining per the manufacturer’s specified intervals. The permissible average power output over a 24 hour period shall not exceed 70% of the ESP rating.
No overload is available.
Not for maintained utility paralleling applications.

5. Standby Power (CAT)
Typical usage 200 hours per year, with a maximum of 500 hours per year with varying loads. Average variable load factor is 70% of Standby rating.
No overload is available.
Not for maintained utility paralleling applications.
Standby
In this application, the generator set is capable of providing emergency backup power at the
nameplate rating for the duration of an outage. The average load factor of a Standby rated
generator set should be no more than 70% of the nameplate rating and applied to varying loads.
A Standby generator set can run for a maximum of 500 hours per year. The normal standby
rating is not for use in utility paralleling applications. For example, a 3 MW standby rated
generator set will provide power for the duration of an outage. It should be run for up to 500 hours
per year and have an average load factor of 2.1 MW.

6. Prime Power (PRP)
Unlimited hours of usage. Average variable load factor is 70% of the Prime power rating. 10% overload No overload is available limited to 1 in 12 hours but not to exceed 25 hours per year. The 10% overload is available in accordance with ISO (2002). Life to overhaul of the engine is dependent on operation as outlined in ISO8528 (2005) and time spent operating above the rating guidelines will reduce the hours to engine overhaul.
Emphasis on 10% Overload for 1 hour only every 12 hrs but not to exceed 25Hrs a year. Sample motor starting capability.

7. Continuous Power (COP)
Unlimited hours of usage. Non-varying load factor is 70% to 100% of the published Continuous Power. Typical peak demand is 100% of continuous rating for 100% of operating hours.

8. Load Factor
Load factor of a generator set is used as one criterion for rating a genset. It is calculated by finding the product of various loads:
% 𝑡𝑖𝑚𝑒= 𝑡𝑖𝑚𝑒 𝑎𝑡 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑙𝑜𝑎𝑑 𝑡𝑜𝑡𝑎𝑙 𝑜𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑡𝑖𝑚𝑒 % 𝑙𝑜𝑎𝑑= 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑙𝑜𝑎𝑑 𝑟𝑎𝑡𝑒𝑑 𝑙𝑜𝑎𝑑
𝐿𝑜𝑎𝑑 𝐹𝑎𝑐𝑡𝑜𝑟=% time x % load

9. Example
For example, assume a facility has a genset rated at 1056 kW and runs it two ours a week. During those two hours, it runs at 950 kW for 1.5 hours. Find the load factor.
% 𝑙𝑜𝑎𝑑= 950𝑘𝑊 1056𝑘𝑊 =0.90 % 𝑡𝑖𝑚𝑒= 90𝑚𝑖𝑛 120𝑚𝑖𝑛 =0.75
𝐿𝑜𝑎𝑑 𝐹𝑎𝑐𝑡𝑜𝑟=0.90 x 0.75
= 67.5%
What rating to choose from?
Which rating will be safe to consider?
Show sample spec sheets…

10. Mode of Operation Island Mode Grid Mode Back Synchronization
•Isochronous: This governor setting allows the generator’s power output to be adjusted based on the system demand.
•Droop Mode : This governor setting allows the generator to be Base Loaded, meaning that the MW output is fixed.

11. t = time
P = power
a = Emergency Standby Power or Prime Power
b = Permissible Average Power output in 24hrperiod(Ppp)
c = Actual Average Power in (Ppa)24hr period
d = stop

12. Sample Scenario
We are operating 3 x 9 MW, 6 kV, 50 Hz gas turbines as part of Utilities power house of a large fertilizer complex which has a total load of 22 MW (mostly large induction motors) The power house is not connected with the public utility, i.e., we are operating our power house in an island mode. At the power house, we also have a load shedding system. In case any gas turbine trips, this load shedding system sheds the fertilizer plant load to bring the total load within the capacity of remaining gas turbine(s). The load shedding system actuates instantly on detection of either turbine trip signal or generator circuit breaker opening. All the gas turbines are being operated in droop mode (droop= 4 %).

13. Sample Scenario
We are operating 3 x 9 MW, 6 kV, 50 Hz gas turbines as part of Utilities power house of a large fertilizer complex which has a total load of 22 MW (mostly large induction motors) The power house is not connected with the public utility, i.e., we are operating our power house in an island mode. At the power house, we also have a load shedding system. In case any gas turbine trips, this load shedding system sheds the fertilizer plant load to bring the total load within the capacity of remaining gas turbine(s). The load shedding system actuates instantly on detection of either turbine trip signal or generator circuit breaker opening. All the gas turbines are being operated in droop mode (droop= 4 %).

14. Sample Scenario
On March 14, 2011, only two gas turbines were running normal (in droop mode) with a plant load of 15 MW. One of the gas turbines tripped on failure of turbine enclosure pressure (which occurred during ventilation fan changeover activity). Tripping of one gas turbine was followed by instant load shedding. However, the 2nd gas turbine also tripped (on generator over-frequency) resulting in total power failure. The plant management afterwards decided to always run all three gas turbines whenever the fertilizer complex total load exceeded 15 MW. The generator under frequency/over-frequency trip settings were: Under-Freq=49,0 Hz / 2.0 s, Over-Freq=51.0 Hz / 0.5 s. power failure.

15. Sample Scenario
On May 10, 2011, we were operating all three gas turbines (in droop mode) with a plant load of 21 MW. Tripping of one gas turbine (on actuation of a gas detector installed in the turbine enclosure) was followed by load shedding and the remaining two gas turbines remained stable, i.e., saved power failure.

16. Sample Scenario
In July 2011, the under-frequency/over-frequency trip settings of the generator were revised to have a wide band in frequency. The revised settings are: Under-Freq=47.5 Hz / 2.0 s, Over-Freq = 52.5 Hz / 3.0 s. Now referring back to the incident of March 14, 2011, the investigation team has recommended to run at least one gas turbine in Isochronous mode to prevent total power failures. You are requested to comment on whether switching one gas turbine to Isochronous mode (while the other machines shall remain in droop mode) shall help in preventing blackouts in case any gas turbine trips.

17. Sample Scenario
In continuation of the above: Suppose a situation when plant total load is 20 MW with following distribution: GT1 (Isochronous mode)=4 MW, GT2 (Droop mode)= 8 MW, GT3 (Droop mode)=8 MW and without any gas turbine tripping, a big block of load like 8 MW is shed (for example, due to a feeder tripping). What would be the response of Isochronous machine in this case? Will it go into reverse power in an attempt to maintain bus frequency? Please keep in mind that there is no communication link between the three turbines for load sharing and the power house is in island mode (i.e., not connected with public utility).

18. De-rate factor and Ambient Condition
Site conditions and power requirements will also play a key role in generator selection. Careful consideration must be given to the environment that the package will be operating in.

19. Voltage
Voltage plays a key role in generator rating and must be considered. In some cases, generator voltage will not match the preferred operating voltage. A voltage regulator can provide voltage adjustment capability, however, when “dialing down” generator voltage, the current will increase for a given rating. This will increase generator heat and may require generator derating. An alternative to generator derating is to use a larger generator to maintain the standard rating. The standard set by NEMA allows a generator to be adjusted up or down by five percent (― 5%) as installed.

20. Environmental Condition
Altitude and temperature most heavily influence engine ratings.
The higher the altitude, the lower the air density. Clean dense air is needed for efficient combustion.
Likewise, an increase in temperature lowers air density.

21. Sample altitude/temperature derate curve
Conductive or abrasive dust drawn in through the cooling fan can be very harmful to the generator. Examples of abrasive dust are: salt, cast iron dust, carbon dust, sand,
powdered graphite, coke dust, lime dust, wood fiber, and quarry dust.
When these foreign particles blow through the generator, they act as
sandpaper scraping away the insulation. These abrasions can
cause an electrical short within the generator. An accumulation of these
materials in the crevices of the insulation system will act as an
insulator or as a moisture attractor. Filters which fit over the unit’s
intake air openings or enclosure ventilation openings can prevent
damage. When using filters, it is important that they be regularly
changed so as not to impede airflow. The use of a generator air
filter will cause the generator to be derated due to higher temperature
rise resulting from reduced cooling airflow. Differential pressure
switches may be available as an option on many generator sets.
Protection from these elements is also derived from the application of
optional Coastal Insulation Protection on main stator windings.
Sample altitude/temperature derate curve
Source:

22. Specifications Required
Voltage
Island
Frequency
Grid
Fuel Type
Sycnhronised
Installation type
Peak Shaving
Application
Peak Lopping
Duty Cycle
Motor Starting
Phase
Summary of Specification. Now are we finish? Can we now size the generator? What is the most important part when sizing generator, the load. Different type of load also have different impact to power generation.

23. The Load Considerations Type of load in terms of application.
PF to be maintained.
Voltage Dip Consideration

24. Type of Loads
Single-phase loads and load imbalance: Single phase loads should be distributed as evenly as possible between the three phases of a three-phase generator set in order to fully utilize generator set capacity and limit voltage imbalance.
Peak loads: Peak loads are caused by loads that cycle on and off—such as welding equipment, medical imaging equipment, or motors. Taking cyclic loads into account can significantly increase the size of the recommended generator set despite painstaking efforts to place loads in a step starting sequence.

25. Type of Loads
Motor loads: Calculating specific motor loads is best handled by sizing software which will convert types of motors into load starting and running requirements.
None-Linear Loads: Due to harmonics caused by electronic rectifiers, larger alternators are required to prevent overheating and to limit system voltage distortion by lowering alternator reactance.
None linear load includes, battery chargers, vfd, medical imaging device that requires 10% voltage dip. In order to protect image quality.
Lighting loads: In addition to lamp wattages, ballast
wattages and starting and running power factors
should be considered.

26. Type of Load
Lighting loads: In addition to lamp wattages, ballast wattages and starting and running power factors should be considered.
Regenerative loads: For loads such as elevators, cranes and hoists, the power source is often relied upon for absorbing power during braking. That is usually not a problem when the utility is supplying power because it can be considered as an infinite power source with many loads. A generator set, in comparison, is able to absorb far less power, especially with no other loads connected. Generally, the regeneration problem can be solved by making sure there are other connected loads which can absorb the regenerative power. Excessive regenerative load can cause a generator set to over-speed and shut down.

27. Voltage Dip Calculation
Here are some reasons why voltage dip calculation is important;
Flickering Light - The human eye is sensitive to slight lighting fluctuations. Even a decrease of 1/2 volt on a 110 volt incandescent bulb is noticeable. A one volt dip, if repeated, becomes objectionable.
Medical Imaging Equipment – to maintain high quality imaging, the voltage dip is limited to 10%.
Equipment Undervoltage protection may trip
Motors to be started may not start as torque is proportional to voltage, induced motors typically designed to start at terminal voltage >80%

28. Why do the calculation?
This calculation is more or less done to verify that the largest motor does not cause system wide problems upon starting. Therefore it should be done after preliminary system design is complete.

29. Step 1. Sg1 = 4000kVA Vg1 = 11,000V X’d = 0.33pu cos ᶿ = 0.85
Size = 500mm2
Length = 50m
R = Ω/km
X = Ω/km
Size = 35mm2
Length = 150m
R = Ω/km
X = Ω/km
Size = 120mm2
Length = 60m
R = Ω/km
X = 0.096Ω/km
STx1 = 1600kV
Vt1 = 11,000V
Vt2 = 400V
Uk = 0.06pu
Pkt = 12700W
tap= 0.0%
PM1 = 250kW
Vg1 = 11,000V
ILRC = A
ILRC/ IFLC = 6.5pu
cos ᶿm = 0.85
cos ᶿs = 0.30
SS1 = 950kVA
Vg1 = 11,000V
cos ᶿ = 0.84
SS1 = 600kVA
Vg1 = 400V
cos ᶿ = 0.80

30. Calculate all Impedances Synchronous Generator Impedance G1
Step 2.
Sg1 = 4000kVA
Vg1 = 11,000V
X’d = 0.33pu
cos ᶿ = 0.85
Calculate all Impedances
Synchronous Generator Impedance G1

31. Cable Impedances C1,C2,C3 Sg1 = 4000kVA Vg1 = 11,000V X’d = 0.33pu
cos ᶿ = 0.85
Cable Impedances C1,C2,C3
Size = 500mm2
Length = 50m
R = Ω/km
X = Ω/km

32. Transformer Impedances TX1
STx1 = 1600kV
Vt1 = 11,000V
Vt2 = 400V
Uk = 0.06pu
Pkt = 12700W
tap= 0.0%

33. Base Load Impedances S1,S2
SS1 = 950kVA
Vg1 = 11,000V
cos ᶿ = 0.84
SS1 = 600kVA
Vg1 = 400V
cos ᶿ = 0.80

34. Motor Impedances M1

35. Computed Impedances

36. Step 3.
Referring the LV Impedances to HV side
415V Standing Load Referred to 11kV Impedance
R = Ω and X = Ω.

37. Step 4.
Equivalent Circuit

38. Step 5.
Calculate Initial Source Voltage

39. Step 6.
Calculate Voltage Dip during stating